Dopant-Free Pyrrolopyrrole-Based (PPr) Polymeric Hole-Transporting Materials for Efficient Tin-Based Perovskite Solar Cells with Stability Over 6000 h

Nucleation-Layer Assisted Quasi-2D Ruddlesden-Popper Tin Perovskite Solar Cells With High Oxygen Stability

Tin (Sn)-based perovskite solar cells (PSCs) are extremely vulnerable to oxygen. Nevertheless, mechanism understanding and fundamental strategies to achieve oxygen-stable Sn-based PSCs are lacking. Here a nucleation-layer assisted (NLA) strategy by forming nucleation layer at the interface of hole transport layer and perovskite to attain highly oxygen-stable quasi-2D Ruddlesden-Popper (RP) Sn-based PSCs is reported. The formation process of nucleation layer consists of washing off the prepared perovskite film and annealing the residue on the substrate, which produces a new substrate for perovskite film fabrication. Such nucleation layer can transform the subsequently deposited perovskite film from a small-n-value dominated wide phase distribution with random crystal orientation into an intermediate-n-value dominated narrow phase distribution with vertical crystal orientation. This nucleation layer also improves the perovskite film morphology with highly coadjacent flake-like grains, leading to reduced grain boundaries and pinholes. The resultant NLA perovskite film shows more efficient carrier transport capability, lower exciton-binding energy, weakened electron-phonon coupling, and significantly decreased oxygen diffusion rate upon oxygen exposure. Consequently, a quasi-2D RP Sn-based PSC with a champion efficiency of 11.18% is obtained. The unencapsulated device preserves 95% of its initial efficiency after a 2700-h oxygen aging test, creating a record oxygen stability for Sn-based PSCs.

Enhancing Stability of Microwave-Synthesized Cs2SnxTi1-xBr6 Perovskite by Cation Mixing

The double-perovskite material Cs2TiBr6 shows excellent photovoltaic potential, making it a promising alternative to lead-based materials. However, its high susceptibility to degradation in air has raised concerns about its practical application. This study introduces an interesting synthesis approach that significantly enhances the stability of Cs2TiBr6 powder. We implemented a gradual cation exchange process by substituting Ti4+ with Sn4+ in the efficient microwave-assisted synthesis method, developing a double perovskite Cs2SnxTi1-xBr6 type. A systematic study of increasing concentration of Sn4+ in Cs2TiBr6 perovskite has been performed to analyze the effect of Sn-doping degree on the chemical and thermal stability of the material and the optical features in both nitrogen and ambient atmospheres, significantly increasing the stability of the material in the air for over a week. Furthermore, introducing Sn4+ results in a more uniform polygonal crystal morphology of the powders and a slight band gap broadening. We show that microwave-assisted synthesis is highly efficient and cost-effective in producing more sustainable lead-free perovskite materials with enhanced stability and desirable electrical characteristics. This work suggests a promising method for synthesizing perovskite materials, opening new routes for scientific research and applications.

Simple I-Shaped Aryl-Based Dyes for Tin Perovskite Solar Cells with Selenophene Core Moiety as Self-Assembled Monolayers on NiOx Using Two-Step Fabrication

Six novel organic small molecules, TPA-Sp-PA (1), TPA-Sp-PE (1E), TPA-T-PA (2), TPA-T-PE (2E), TPA-P-PA (3) and TPA-P-PE (3E) are developed and applied to NiOx films as self-assembled monolayers (SAMs) for tin perovskite solar cells (TPSCs). The linker between acceptor (phosphonic acid (PA) or phosphonic ester (PE)) and donor (triphenylamine (TPA)) plays an important role in facilitating the growth of high-quality perovskite films using a two-step method. Three different types of linkers, phenyl ring (P), thiophene (T), and selenophene (Sp), are studied, for which the Sp-based SAMs provide the best device performance with TPA-Sp-PE (1E) achieving a PCE 8.7%, and its acidic analog, TPA-Sp-PA (1), reaching a maximum PCE of 8.3%. Single crystal structures of TPA-Sp-PE (1E) and TPA-T-PE (2E) are successfully obtained, with the expectation that a uniform SAM would form on the NiOx/ITO substrate. The research introduces a novel approach to enhance TPSC performance by integrating organic SAMs with NiOx HTMs, offering a promising avenue for future progress in TPSC technology through a two-step fabrication technique.

Halogen Radical-Activated Perovskite-Substrate Buried Heterointerface for Achieving Hole Transport Layer-Free Tin-Based Solar Cells with Efficiencies Surpassing 14

Sn-based perovskites have emerged as one of the most promising environmentally-friendly photovoltaic materials owing to their low toxicity and exceptional optoelectronic properties. Nonetheless, the low-cost production and stable operation of Sn-based perovskite solar cells (PSCs) are still largely limited by the costly hole transport materials and the under-optimized interfaces between hole transport layer (HTL) and Sn perovskite layer. Here, we innovatively developed a chlorine radical chemical bridging (Cl-RCB) strategy that enabled to remove the HTL and optimize the indium tin oxide (ITO)/perovskite heterointerface for constructing high-performance Sn-based PSCs with simplified structures. The key is to modify the commercially-purchased ITO electrode with highly active chlorine radicals that could effectively mitigate the surface oxygen vacancies, alter the chemical constitutions, and favorably down-shifted the work function of ITO surface to be close to the valence band of perovskites. As a result, the interfacial energy barrier has been largely reduced by 0.20 eV and the interfacial carrier dynamics have been optimized at the ITO/perovskite heterointerface. Encouragingly, the efficiency of HTL-free Sn-based PSCs has been enhanced from 6.79 % to 14.20 %, which is on par with the state-of-the-art conventional HTL-containing counterparts (normally >14 % efficiency) and representing the record performance for the Sn perovskite photovoltaics in the absence of HTL. Notably, the target device exhibited enhanced stability for up to 2000 h. The Cl-RCB strategy is also versatile to be used in Pb-based and mixed Sn-Pb HTL-free PSCs, achieving efficiencies of 22.27 % and 21.13 %, respectively, all representing the advanced device performances for the carrier transport layer-free PSCs with simplified device architectures.

Bithiophene Imide-Based Self-Assembled Monolayers (SAMs) on NiOx for High-Performance Tin Perovskite Solar Cells Fabricated Using a Two-Step Approach

Three new bithiophene imide (BTI)-based organic small molecules, BTI-MN-b4 (1), BTI-MN-b8 (2), and BTI-MN-b16 (3), with varied alkyl side chains, were developed and employed as self-assembled monolayers (SAMs) applied to NiOx films in tin perovskite solar cells (TPSCs). The NiOx layer has the effect of modifying the hydrophilicity and the surface roughness of ITO for SAM to uniformly deposit on it. The side chains of the SAM molecules play a vital role in the formation of a high-quality perovskite layer in TPSCs. The single crystal structure of BTI-MN-b8 (2) was successfully obtained, indicating that a uniform SAM can be formed on the NiOx/ITO substrate with an appropriate size of the alkyl side chain. By combining BTI-MN-b8 (2) with NiOx, a maximum PCE of 8.6% was achieved. The TPSC devices utilizing the NiOx/BTI-MN-b8 configuration demonstrated outstanding long-term stability, retaining ∼80% of their initial efficiency after 3600 h. Comprehensive characterizations, including thermal, optical, electrochemical, and morphological analyses, alongside photovoltaic evaluation, were carried out thoroughly. This study presents a pioneering strategy for improving TPSC performance, highlighting the efficacy of combining organic SAMs with NiOx as the HTM and offering a promising pathway for future advances in TPSC technology using a two-step fabrication approach.

Triphenylamine-Based Y-Shaped Self-Assembled Monolayers for Efficient Tin Perovskite Solar Cells

Triphenylamine-based Y-shaped organic sensitizers, specifically TPA-MN (1), TPA-CA (2), TPAT-MN (3), and TPAT-CA (4), are synthesized and utilized as p-type self-assembled monolayers (SAMs) for tin-based perovskite solar cells (TPSCs). These SAMs are developed using low-cost starting materials, primarily from triphenylamine (TPA) components. An extensive analysis is conducted to examine the crystalline, morphological, thermal, optical, electrochemical, and optoelectronic characteristics of SAMs 1-4, and the results are compared. A two-step method is employed to successfully develop tin perovskite layers on all four SAM surfaces. The resulting devices demonstrates PCE in the following order: TPAT-CA (8.1%) > TPAT-MN (6.1%) > TPA-MN (5.0%) > TPA-CA (4.2%). The TPAT-CA molecule, which contains a thiophene spacer, performed better than the other three SAMs in terms of rapid hole extraction rate, high hole mobility, and retarded charge recombination. Consequently, SAM TPAT-CA exhibited the highest device performance with excellent stability over time, retaining ≈90% from the beginning values after storage for 3000 h. The innovative Y-shaped SAMs describe in this study, characterized by their simple and efficient design, have the potential to contribute significantly to the advancement of perovskite photovoltaics, particularly in the development of cost-effective TPSC technology.

From Wide-Bandgap to Narrow-Bandgap Perovskite: Applications from Single-Junction to Tandem Optoelectronics

As perovskite solar device is burgeoning photoelectronic device, numerous studies to optimize perovskite solar device have been demonstrated. Amongst various advantages from perovskite light absorbing layer, attractive property of tunable bandgap allowed perovskite to be adopted in many different fields. Easily tunable bandgap property of perovskite opened the wide application and to get the most out of its potential, many researchers contributed as well. By precursor composition engineering, narrow bandgap with bandgap of less than 1.4 eV and wide bandgap with bandgap of more than 1.7 eV were achieved. Optimization of both narrow and wide bandgap perovskite solar cell could pave the way to all-perovskite tandem solar cell which is combination of top cell with wide bandgap and bottom cell with narrow bandgap. This review highlights numerous efforts to advance device performance of both narrow and wide bandgap perovskite solar cell and how they challenged the issues. And finally, efforts to operate and utilize all-tandem perovskite device in real world will be discussed.

Tailored Polymer Hole-Transporting Materials with Multisite Passivation Functions for Effective Buried-Interface Engineering of Inverted Quasi-2D Perovskite Solar Cells

Although quasi-2D Ruddlesden‒Popper (RP) perovskite exhibits advantages in stability, their photovoltaic performance are still inferior to 3D counterparts. Optimizing the buried interface of RP perovskite and suppress energetic losses can be a promising approach for enhancing efficiency and stability of inverted quasi-2D RP perovskite solar cells (PSCs). Among which, constructing polymer hole-transporting materials (HTMs) with defect passivation functions is of great significance for buried-interface engineering of inverted quasi-2D RP PSCs. Herein, by employing side-chain tailoring strategy to extend the π-conjugation and regulate functionality of side-chain groups, target polymer HTMs (PVCz-ThSMeTPA and PVCz-ThOMeTPA) with high mobility and multisite passivation functions are achieved. The presence of more sulfur atom-containing groups in side-chain endows PVCz-ThSMeTPA with increased intra/intermolecular interaction, appropriate energy level, and enhanced buried interfacial interactions with quasi-2D RP perovskite. The hole mobility of PVCz-ThSMeTPA is up to 9.20 × 10-4 cm2 V-1 S-1. Furthermore, PVCz-ThSMeTPA as multifunctional polymer HTM with multiple chemical anchor sites for buried-interface engineering of quasi-2D PSCs can enable effective charge extraction, defects passivation, and perovskite crystallization modulation. Eventually, the PVCz-ThSMeTPA-based inverted quasi-2D PSC achieves a champion power conversion efficiency of 22.37%, which represents one of the highest power conversion efficiencies reported to date for quasi-2D RP PSCs.

Ternary Passivation for Enhanced Carrier Transport and Recombination Suppression in Highly Efficient Sn-Based Perovskite Solar Cells

The exploration of nontoxic Sn-based perovskites as a viable alternative to their toxic Pb-based counterparts has garnered increased attention. However, the power conversion efficiency of Sn-based perovskite solar cells lags significantly behind their Pb-based counterparts. This study presents a ternary passivation strategy aimed at enhancing device performance, employing [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM), poly(3-hexylthiophene) (P3HT), and indene C60 bisadduct (ICBA). These components play crucial roles in managing energy levels and enhancing carrier transportation, respectively. The results reveal that the introduction of the ternary system leads to improvements in carrier collection and transportation, accompanied by a suppression of the recombination process. Ultimately, the champion device achieves a remarkable performance with an efficiency of 14.64%. Notably, the device also exhibits robust operational and long-term stored stability.

Innovative Materials for High-Performance Tin-Based Perovskite Solar Cells: A Review

With the rapid development of lead-based perovskite solar cells, tin-based perovskite solar cells are emerging as a non-toxic alternative. Material engineering has been an effective approach for the fabrication of efficient perovskite solar cells. This paper summarizes the novel materials used in tin-based perovskite solar cells over the past few years and analyzes the roles of various materials in tin-based devices. It is found that self-assembling materials and fullerene derivatives have shown remarkable performance in tin-based perovskite solar cells. Finally, this article discusses design strategies for new materials, providing constructive suggestions for the development of innovative materials in the future.

Functionalized Thienopyrazines on NiOx Film as Self-Assembled Monolayer for Efficient Tin-Perovskite Solar Cells Using a Two-Step Method

Three functionalized thienopyrazines (TPs), TP-MN (1), TP-CA (2), and TPT-MN (3) were designed and synthesized as self-assembled monolayers (SAMs) deposited on the NiOx film for tin-perovskite solar cells (TPSCs). Thermal, optical, electrochemical, morphological, crystallinity, hole mobility, and charge recombination properties, as well as DFT-derived energy levels with electrostatic surface potential mapping of these SAMs, have been thoroughly investigated and discussed. The structure of the TP-MN (1) single crystal was successfully grown and analyzed to support the uniform SAM produced on the ITO/NiOx substrate. When we used NiOx as HTM in TPSC, the device showed poor performance. To improve the efficiency of TPSC, we utilized a combination of new organic SAMs with NiOx as HTM, the TPSC device exhibited the highest PCE of 7.7 % for TP-MN (1). Hence, the designed NiOx/TP-MN (1) acts as a new model system for the development of efficient SAM-based TPSC. To the best of our knowledge, the combination of organic SAMs with anchoring CN/CN or CN/COOH groups and NiOx as HTM for TPSC has never been reported elsewhere. The TPSC device based on the NiOx/TP-MN bilayer exhibits great enduring stability for performance, retaining ~80 % of its original value for shelf storage over 4000 h.

Counter-Doping Effect by Trivalent Cations in Tin-Based Perovskite Solar Cells

Tin (Sn) -based perovskite solar cells (PSCs) normally show low open circuit voltage due to serious carrier recombination in the devices, which can be attributed to the oxidation and the resultant high p-type doping of the perovskite active layers. Considering the grand challenge to completely prohibit the oxidation of Sn-based perovskites, a feasible way to improve the device performance is to counter-dope the oxidized Sn-based perovskites by replacing Sn2+ with trivalent cations in the crystal lattice, which however is rarely reported. Here, the introduction of Sb3+, which can effectively counter-dope the oxidized perovskite layer and improve the carrier lifetime, is presented. Meanwhile, Sb3+ can passivate deep-level defects and improve carrier mobility of the perovskite layer, which are all favorable for the photovoltaic performance of the devices. Consequently, the target devices yield a relative enhancement of the power conversion efficiency (PCE) of 31.4% as well as excellent shelf-storage stability. This work provides a novel strategy to improve the performance of Sn-based PSCs, which can be developed as a universal way to compensate for the oxidation of Sn-based perovskites in optoelectronic devices.

Colloidal Zeta Potential Modulation as a Handle to Control the Crystallization Kinetics of Tin Halide Perovskites for Photovoltaic Applications

Tin halide perovskites (THPs) have demonstrated exceptional potential for various applications owing to their low toxicity and excellent optoelectronic properties. However, the crystallization kinetics of THPs are less controllable than its lead counterpart because of the higher Lewis acidity of Sn2+, leading to THP films with poor morphology and rampant defects. Here, a colloidal zeta potential modulation approach is developed to improve the crystallization kinetics of THP films inspired by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. After adding 3-aminopyrrolidine dihydro iodate (APDI2) in the precursor solution to change the zeta potential of the pristine colloids, the total interaction potential energy between colloidal particles with APDI2 could be controllably reduced, resulting in a higher coagulation probability and a lower critical nuclei concentration. In situ laser light scattering measurements confirmed the increased nucleation rate of the THP colloids with APDI2. The resulting film with APDI2 shows a pinhole-free morphology with fewer defects, achieving an impressive efficiency of 15.13 %.

Diselenophene-Dithioalkylthiophene Based Quinoidal Small Molecules for Ambipolar Organic Field Effect Transistors

This work presents a series of novel quinoidal organic semiconductors based on diselenophene-dithioalkylthiophene (DSpDST) conjugated cores with various side-chain lengths (-thiohexyl, -thiodecyl, and -thiotetradecyl, designated DSpDSTQ-6, DSpDSTQ-10, and DSpDSTQ-14, respectively). The purpose of this research is to develop solution-processable organic semiconductors using dicyanomethylene end-capped organic small molecules for organic field effect transistors (OFETs) application. The physical, electrochemical, and electrical properties of these new DSpDSTQs are systematically studied, along with their performance in OFETs and thin film morphologies. Additionally, the molecular structures of DSpDSTQ are determined through density functional theory (DFT) calculations and single-crystal X-ray diffraction analysis. The results reveal the presence of intramolecular S (alkyl)···Se (selenophene) interactions, which result in a planar SR-containing DSpDSTQ core, thereby promoting extended π-orbital interactions and efficient charge transport in the OFETs. Moreover, the influence of thioalkyl side chain length on surface morphologies and microstructures is investigated. Remarkably, the compound with the shortest thioalkyl chain, DSpDSTQ-6, demonstrates ambipolar carrier transport with the highest electron and hole mobilities of 0.334 and 0.463 cm2 V-1 s-1 , respectively. These findings highlight the excellence of ambipolar characteristics of solution-processable OFETs based on DSpDSTQs even under ambient conditions.

On the Durability of Tin-Containing Perovskite Solar Cells

Tin (Sn)-containing perovskite solar cells (PSCs) have gained significant attention in the field of perovskite optoelectronics due to lower toxicity than their lead-based counterparts and their potential for tandem applications. However, the lack of stability is a major concern that hampers their development. To achieve the long-term stability of Sn-containing PSCs, it is crucial to have a clear and comprehensive understanding of the degradation mechanisms of Sn-containing perovskites and develop mitigation strategies. This review provides a compendious overview of degradation pathways observed in Sn-containing perovskites, attributing to intrinsic factors related to the materials themselves and environmental factors such as light, heat, moisture, oxygen, and their combined effects. The impact of interface and electrode materials on the stability of Sn-containing PSCs is also discussed. Additionally, various strategies to mitigate the instability issue of Sn-containing PSCs are summarized. Lastly, the challenges and prospects for achieving durable Sn-containing PSCs are presented.

Aspartate all-in-one doping strategy enables efficient all-perovskite tandems

All-perovskite tandem solar cells hold great promise in surpassing the Shockley-Queisser limit for single-junction solar cells1-3. However, the practical use of these cells is currently hampered by the subpar performance and stability issues associated with mixed tin-lead (Sn-Pb) narrow-bandgap perovskite subcells in all-perovskite tandems4-7. In this study, we focus on the narrow-bandgap subcells and develop an all-in-one doping strategy for them. We introduce aspartate hydrochloride (AspCl) into both the bottom poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) and bulk perovskite layers, followed by another AspCl posttreatment. We show that a single AspCl additive can effectively passivate defects, reduce Sn4+ impurities and shift the Fermi energy level. Additionally, the strong molecular bonding of AspCl-Sn/Pb iodide and AspCl-AspCl can strengthen the structure and thereby improve the stability of Sn-Pb perovskites. Ultimately, the implementation of AspCl doping in Sn-Pb perovskite solar cells yielded power conversion efficiencies of 22.46% for single-junction cells and 27.84% (27.62% stabilized and 27.34% certified) for tandems with 95% retention after being stored in an N2-filled glovebox for 2,000 h. These results suggest that all-in-one AspCl doping is a favourable strategy for enhancing the efficiency and stability of single-junction Sn-Pb perovskite solar cells and their tandems.

Highly Efficient HTM-Free Tin Perovskite Solar Cells with Outstanding Stability Exceeding 10000 h

The bottleneck in the rapid development of tin-based perovskite solar cells (TPSCs) is the inherent chemical instability. Although this is being addressed continuously, the device performance has not improved further due to the use of PEDOT:PSS as the hole-transport material (HTM), which has poor long-term stability. Herein we have applied commercial ITO nanoparticles over ITO glass substrates and altered the surface chemistry of the ITO electrode via a simple two-step thermal annealing, followed by a UV-ozone treatment. These surface-modified ITO electrodes display promising interfacial characteristics, such as a suitable band alignment owing to significantly reduced surface carbon contamination, increased In-O bonding, and reduced oxygen vacancies, that enabled fabrication of an HTM-free TPSC device according to a two-step method. The fabricated device possessed an outstanding power conversion efficiency (PCE) of 9.7%, along with a superior long-term stability by retaining over 90% of the initial PCE upon shelf storage in a glovebox for a period of over 10000 h. The application of ITO nanoparticles led to effective interfacial passivation, whose impacts on the long-term durability were assessed using electrochemical impedance spectroscopy, time-resolved photoluminescence decay profiles, and femtosecond transient absorption spectroscopy techniques.